Ask The Experts

I’m looking for a one- or two-part silicone adhesive with shear strength of 4–5 MPa. Can you suggest a material?

mpmn — Wed, 01 Sep 2010 17:37:34 +0000

Dow Corning offers Silastic MDX4-4210, a two-component silicone adhesive that has a tensile strength of 5 MPa. NuSil offers MED-1000 one-part silicone adhesive and MED2-4013 two-part silicone adhesive, which have similar strength characteristics. Momentive Performance Materials, Rhodia, and Wacker offer alternatives as well. These adhesives are listed as tensile-strength materials. Their operating data are acquired by molding a dog-bone tensile bar and pulling it 180° apart until it breaks. The breaking point determines the tensile strength. Bonding two pieces of a substrate is tensile shear strength, but adhesion depends on the substrate itself as well as the surface area. Most vendors cannot test all combinations and substrates, so evaluation of these candidate adhesives determines if they will reach 4–5 MPa, as you require.

I recommended these materials based on their tensile strength. If a material has a lower tensile strength, it might tear cohesively in the application, leaving silicone adhesive on both substrates. By selecting a high-tensile-strength material, the cohesive strength of the adhesive will be above your minimum value, so that you can focus on the actual adhesion of the silicone adhesive to the substrate.

We are looking for a room-temperature medical-grade cure medium for high-viscosity epoxy. The parts we are bonding are made from Pebax (55D) and feature an epoxy surface. The ideal set time is less than 1 hour, and the cure time should be less than 24 hours at room temperature.

mpmn — Tue, 31 Aug 2010 23:21:33 +0000

I would initially start looking at either Epoxy Technology’s Epo-Tek 730 or Henkel’s Hysol M-21 HP epoxy adhesives for medical device applications. This is a good place to start, but there are other products on the market as well. Pebax is a copolymer of nylon and urethane-type polymers. The 55D is a mid-range polymer, while the 72D is more nylonlike and 32D is more urethanelike.

I am searching for a glue that will bond glass to plastic and that dries clear.

mpmn — Tue, 31 Aug 2010 23:13:55 +0000

There are a number of clear adhesives that are suitable for bonding glass to plastic and that remain clear and colorless. Dymax 429 or 4-20418 are candidates that cure with UV light in seconds to form a strong bond between multiple substrates. Epotek 353ND is a clear two-part epoxy that would also work well. These few options might get you started looking in the right direction.

A key criterion for selecting the right material for your application is to identify how many parts you will make per day, per month, and per year and what type of process you can envision. Is high-speed UV curing the best option, or should you perhaps consider using the benefits of light-curing materials, which cure “on-demand”? Such adhesives allow you to align the parts, and once they are aligned, you shine light on them for a few seconds, locking them in place. Or are you more comfortable using a two-part material that needs to be mixed and degassed prior to use to avoid air bubbles? Some epoxies are available in 1:1 mix-ratio cartridges, allowing you to dispense through a static mixer system and easing the formation of air bubbles. Dymax ER1196/CT1196 is an 8000 cP two-component epoxy available in a 1:1 mix.

Another factor in selecting the right material is to determine what viscosity will work best for your part or application. To put this in perspective, water = 1 cP, honey = 10,000 cP, Dymax 429 = 2,500 cP, and Dymax 4-20418 = 450 cP.

We have an application in which a ½ × ½ x 1-in. bar is inserted to a depth of ½ in. in a ½ × ½-in. hole machined on a flat bar. Both parts are made of anodized 7075 T6 aluminum alloy. The inserted bar receives intermittent torque around the long axis. We are using common two-component epoxy adhesive manufactured by DuPont. After assembling, the parts are heated to 260°F for sterilization purposes. The results are rather poor: After being in use for a short period of time, the parts become loose and separate. Is there a product that we can use as a substitute?

mpmn — Wed, 25 Aug 2010 19:58:34 +0000

There are a couple of choices that come to mind based on your application. Several epoxy options can survive these temperatures and exhibit good adhesion. Dymax ER1196/CT1196 has shown good adhesion up to 285°F. Epotek 353ND is very popular for these types of medical applications. And Epotek 375, which is classified as a high-temperature epoxy, can also be used. Basically, any epoxy that is designed for rugged or ruggedizing applications will be more flexible (i.e., less brittle) so that it can absorb the torsional strain.

Another option is high-temperature epoxies that move the glass transition temperature (Tg) above the sterilization temperature. A typical epoxy can have Tg values in the 80° to 120°C range. When it undergoes sterilization at 260°F/126°C, it crosses this glass transition temperature. An epoxy with a process temperature higher than the Tg value will exhibit more rubberlike behavior, while an epoxy with a process temperature below the Tg value will exhibit more glasslike behavior. Moving the part through this transition (change in temperature) causes stress in the bond line. A high-temperature epoxy should have a Tg above the sterilization temperature so that the bonded part does not have to experience this transition.

We are conducting taste experiments with sucrose, saline, quinine, citric acid, and MSG solutions and are exploring different methods to prepare or sterilize the solutions. We will then have the contents tested for various contaminants at 30-, 60-, and 90-day intervals. However, our analytical lab is not sure what standard to use as a cut point for stopping the analysis. Any suggestions?

mpmn — Tue, 24 Aug 2010 23:08:45 +0000

There are at least four issues involved here: How long does the product maintain the desired organoleptic properties? How long will the packaging maintain the desired state (carbonation, sterility [barrier?]? How long are the preservatives supposed to be effective? What is the desired or labeled shelf life?

The study should be as long as necessary to cover the shortest of the product requirements. For example, if the product changes color, taste, etc. at 180 days, it should be tested to 180+ days. If the package maintains product integrity for only 120 days, 120+ days should suffice. If there is any possibility that the product will be on the shelf longer than the organoleptic or microbial data support, shelf-life labeling for the validated interval would be appropriate. All studies should be longer than the labeled shelf life or longer than the shortest interval that the product safety test data support.

I have LCP from Vectra, E130i, 30% glass filled, and I would like to find a surface preparation. I am pouring a flowable RTV 734 from Dow Corning over it.

mpmn — Tue, 10 Aug 2010 19:03:10 +0000

To increase adhesion to the Vectra LCP or similar materials, you have a few options:

1. Use an alcohol wipe with a 70:30 solution of IPA:water to clean the surface and put a uniform layer of -hydroxy groups onto the glass beads. As the glass ages, the humidity in the air deposits a buildup of hydroxyl units onto the surface. They are held in place by weak van der Waals forces. When an adhesive is placed on top, it is easy to remove these weak boundary layers. By removing them and starting fresh, you will achieve better adhesion.

2. Surface abrasion will increase the surface area and create a mechanical interlock, both yielding better adhesion to the substrate.

3. Dow Corning offers a few different primer systems, such as Dow Corning 1200. The primer can be applied by brush or spray. Allow the solvent to evaporate and then apply the adhesive to the bond line.

4. An air plasma treatment can also increase adhesion. Enercon and other companies offer gas or flame plasma treatment systems that can deposit bonding groups onto a substrate surface, enabling better chemical bonds to the surface.

5. Sometimes, a combination of some of the above options will have good results—in other words, surface abrasion with an alcohol wipe.

I am trying to bond a small PVC jacketed ribbon cable wire to a small piece of 17-4 stainless steel. The problem is, the bonding area is very small. We have been using a Cotronics two-part epoxy, which gets very hard but does not seem to be very tough. We are getting wire to substrate delaminations. Should we perhaps try a flexibilized two-part epoxy? The use temperature would be considered ambient, and there would be no moisture issues.

mpmn — Tue, 10 Aug 2010 16:56:41 +0000

Where there are a number of different options available, I would initially start with Dymax 846-GEL/501-E acrylated urethane, with other options being 3M ScotchWeld 2216, Hysol 9462 toughened epoxy, or Hysol U-05FL urethane, to name just a few. One thing to keep in mind is the failure mode of the current material and other eventual materials. Is the adhesive sticking to one side versus the other, as in the case of PVC or stainless steel? This would indicate adhesive failure. Or is it sticking to both, indicating cohesive failure? Or are you getting a mixed result—adhesion and cohesion failure? Are you testing in a shear mode (pulling the materials along the bond line in opposite directions), or in more of a peel mode (a 90° peel)? Evaluating the bond line for these issues will help you select adhesives for your application.

What is your understanding of current requirements for demonstrating biocompatibility in product contact packaging?

mpmn — Thu, 05 Aug 2010 17:59:43 +0000

ASTM 2475-05 describes the current requirements for demonstrating biocompatibility in product contact packaging. The selection of which tests to choose depends on the interaction of the packaging and the device. Typically, the packaging material contacts only the medical device and is considered indirect contact to the patient. This condition therefore reduces the level of testing required. If the device involves a fluid or drug that will come in contact with the packaging, more-stringent testing is be required. Consultation with a biocompatibility testing firm is recommended.

The following table provided by SGS (Rutherford, NJ) summarizes the suggested testing for different medical device packaging materials.

My question relates to the effect of radiation (gamma, E-beam) on a silicone material, specifically LSRs and HCRs. What technologies are available to combat radiation-induced methyl-to-methyl cross-linking between surfaces in contact with each other? The end result is that molded parts can stick to themselves. Are there any additives or processes that could be employed to minimize or eliminate the free radical generation or the effects of the free radicals?

mpmn — Thu, 05 Aug 2010 17:53:05 +0000

Sticking parts can definitely be an issue, especially with lower-durometer molded parts. The first thing I would suggest trying is to perform a postcure at ~350°–400°F for two to four hours after molding. This finishes most of the cross-linking that may be occurring after molding and is occurring during your gamma-radiation period in your application. An additive that helps with lubricity but may also be an issue in your application is a filler to add to the silicone. I know that Zonyl (Teflon), which is sometimes used to create lubricity within molded silicone components, would reduce sticking and cross-linking between two like materials. Generally a mixed ratio of ~10–15% Zonyl by weight is a sufficient loading. But I would try the postcure first.

If we use phosphorescent pigment glow color products from UMC Corp. with your silicone, can the silicone be placed in the mouth? Will the product be FDA compliant? Would it have to be a two-part product in which only the silicone is placed in the mouth while the rest of the phosphorescent pigment glow silicone color remains outside the mouth, like a pacifier?

mpmn — Mon, 02 Aug 2010 18:17:11 +0000

As a silicone molder and extruder, Sil-Pro uses a variety of silicones from multiple manufacturers, including Dow Corning, Nusil, Momentive, and Wacker, to name a few. All of these companies offer materials that are approved for contact with body fluids or for use in the mouth, and all would have FDA compliance/approval for use. However, since I am not aware of UMC Corp. or its glow color products, I am not familiar with the company’s certifications. Sil-Pro uses pigments from Nusil and other companies that are approved for use in short-term implants or for applications that are placed in the mouth.